Anatomical and chemical defenses of conifer bark against bark beetles and other pests

Summary Conifers are long‐lived organisms, and part of their success is due to their potent defense mechanisms. This review focuses on bark defenses, a front line against organisms trying to reach the nutrient‐rich phloem. A major breach of the bark can lead to tree death, as evidenced by the millions of trees killed every year by specialized bark‐invading insects. Different defense strategies have arisen in conifer lineages, but the general strategy is one of overlapping constitutive mechanical and chemical defenses overlaid with the capacity to up‐regulate additional defenses. The defense strategy incorporates a graded response from ‘repel’, through ‘defend’ and ‘kill’, to ‘compartmentalize’, depending upon the advance of the invading organism. Using a combination of toxic and polymer chemistry, anatomical structures and their placement, and inducible defenses, conifers have evolved bark defense mechanisms that work against a variety of pests. However, these can be overcome by strategies including aggregation pheromones of bark beetles and introduction of virulent phytopathogens. The defense structures and chemicals in conifer bark are reviewed and questions about their coevolution with bark beetles are discussed. Contents Summary 353 I. Introduction 354 II. Integrating defense strategies 354 III. Constitutive defense systems: first tier of defense 356 IV. Inducible defense systems: second tier of defense 363 V. Genetics and physiology of bark defense 365 VI. Bark beetles: diminutive but deadly 367 VII. The arms race: coevolution of conifer defense and bark beetle strategies 367 VIII. Bark‐beetle‐vectored blue‐stain fungi: multiple attacks against multiple defenses 369 IX. Conclusions 370 Acknowledgements 371 References 371

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